Various types of electrical measuring instruments are lowered into boreholes by an electric cable to investigate the subsurface earth formations penetrated by those boreholes. To carry out these measurements, these logging tools are equipped with varying numbers of electrodes depending on the type of measurements to be made. One typical logging tool includes a centrally-located electrode and several pairs of electrodes which are spatially disposed along the cylindrical tool body so as to be located symmetrically with respect to the central electrode. This type of logging tool is described, for example, in U.S. Pat. No. 3,772,589.
These logging tools generally include a central metal mandrel to give mechanical rigidity to the tool body. A sheath of electrically-insulating material encases the mandrel. Metal electrodes of hollow cylinders or annular bands having an inner diameter substantially equal to the outer diameter of the insulating sheath are mounted along the mandrel and separated from one another by insulated annular spacers having a thickness substantially equal to that of the electrodes. Longitudinal grooves on the interior surfaces of the electrodes and spacers define passages for receiving wires that are respectively connected to the electrodes and grouped together at one end of the tool for connection to a common plug.
Prior-art tools have sometimes been found to be unreliable due to the difficulty of maintaining adequate electrical insulation between the various components. Part of such problems come about since the insulating sheath on the tool mandrel must be relatively thin (about 1-mm) so as to not significantly increase the overall diameter of the tool body. It will be recognized, of course, that when the tool is disassembled for repair of some malfunction it is quite easy to damage this sheath. When the tool is disassembled, it is necessary to remove the spacers and sometimes even the electrodes. This disassembly is not always easy, however, since drilling mud has usually infiltrated into the tool body and has become caked. Moreover, certain parts may have become rusted and thereby further complicate the disassembly. In addition, when the insulating sheath itself has become defective, its replacement can be carried out only in a shop having suitable tools. It is also possible to create electrical insulation defects during the reassembly of the logging tool such as when the insulating spacers are being positioned. In fact, should a connection wire be poorly positioned in its groove, it can be pinched and damaged.
The repair of prior-art logging tools of this type is not very convenient; and, in most cases, such repairs usually cannot be performed directly on the drilling site. Usually the logging operation must be interrupted or postponed to bring the tool into a shop, thus involving a loss of time. Drilling downtime is costly and it will be appreciated that the possibility of having logging tools capable of being rapidly repaired on the drilling site clearly represents an unquestionable technical and economic advantage.